Frequency selective components such as resonators are important for many electronic products requiring stable frequency signals or ability to discriminate between signals based on frequency diversity. These functions are difficult to reliably and repeatably realize in monolithic form together with other microelectronic components such as transistors, diodes and the like.
One approach to realizing frequency selective functions employs a mass allowed to vibrate in one or more dimensions (e.g., a pendulum). Such a mass is conveniently realized as a thin membrane supported at critical points, for example, peripherally or alternatively along one edge or end, forming a thin resonator structure. Such structures provide clearly defined mechanical resonances having significant utility, for example, as filters and as frequency stabilizing feedback elements in oscillator circuits. These structures have the advantages of being extremely compact and of providing narrow bandwidth (i.e., high quality factor) frequency selection components that are light weight and which do not require adjustment over the life of the component.
A significant drawback of previous thin resonators has been the need to fabricate a free-standing thin film membrane. Typically, this is effected by forming a sacrificial layer followed by deposition of the membrane. The sacrificial layer is then selectively removed, leaving a self-supporting layer.
An alternative approach involves forming a cantilevered beam capacitively coupled to adjacent structures (e.g., a conductor placed beneath the beam). The beam is free to vibrate and has one or more resonance frequencies. Disadvantages of these approaches include need to form free-standing structures and also a tendency of the beam to "stick" to adjacent structures if or when the beam comes into contact therewith.
Problems encountered with such devices include temperature sensitivity of the device resulting in temperature-induced shifts in the center frequency of the frequency selection component. Piezoelectric materials having higher electromechanical coupling coefficients (e.g., LiNbO.sub.3, LiTaO.sub.3, AlPO.sub.4, BiGeO.sub.20, BiSiO.sub.20 and the like) are preferred for some applications but tend to have larger temperature coefficients and so tend to have frequency selection properties that are more strongly temperature dependent. This is more of a problem for applications requiring closer frequency tolerances because the desired frequency characteristics must be maintained more closely over the operating temperature range.
An additional problem that may occur for some applications is that the Q or quality factor of the material(s) employed in the resonator may preclude providing the required bandwidth and insertion loss in the completed structure. Generally, narrow bandwidths together with low insertion losses require high Q materials. Deposited thin-film layers of piezoelectric materials tend to have poorer (i.e., lower) quality factors than the same materials prepared by other techniques (e.g., single-crystal piezoelectric materials) and this may limit the achievable bandwidth. Additionally, employing lossy materials for electrodes (e.g., Au, Ag, Pb etc.) reduces the overall Q of the resonator structure while use of low acoustic loss materials (e.g., Al and alloys thereof) has less of an impact on the Q of the resonator structure. Accordingly, the bandwidth requirements for some applications may preclude use of some materials in the resonator and require the use of other materials or particular material preparation techniques. The required materials may have temperature characteristics dictating need for temperature stabilization of the overall resonator structure.
Many applications require robust, light-weight devices to be realized in small form factor and to consume as little electrical power as possible while operating over a broad range of temperatures. For example, satellite communications apparatus have stringent power requirements and also must operate over a broad temperature range. This example also places a premium on size, weight and reliability.
What are needed are apparatus and methods for forming apparatus wherein the apparatus provides a thin acoustic resonator having solid mechanical support and including temperature-stable, narrow-bandwidth frequency selection characteristics together with low power consumption requirements.